Well,

If you are talking about a human, then it means "somebody" e.g. I hear somebody talking >> انا أسمع أحد يتحدث

If you are talking about things either real or incorporeal things, then it means "one" but not the number 1, e.g.

- "That's one reason" >> هذا أحد الأسباب

- but "That's reason number one" >> هذا السبب رقم واحد

When adding ال to the word أحد it becomes الأحد which means Sunday

.

The word واحد in Standard Arabic means the number 1

أحد 

It Has many meanings, for example if you hear someone at the room
you say " Who's there" in Arabic you could say " هل هناك أحد بالغرفة"
and  he or she maybe will say nobody is there
لا يوجد أحد هناك

It's also a day "Saturday"  يوم الأحد 

Also you could find it in the Quran at the Sura Ikhlas
"Say, "He is Allah, [who is] One "
قل هو الله أحد

------------

واحد
Is just number one 1

أحد-واحد

أحد-واحد

in simple way

أحد   u can't increase it because he/it is unique .

واحد u can increase it because there is more than one in his/it kind